Chlorine adsorption and diffusion on Cu(111)

The adsorption, reaction and surface diffusion of chlorine on Cu(111) has been studied in the temperature range 110 to 900 K using low energy electron diffraction, Auger electron spectroscopy (AES), temperature programmed thermal desorption and electron stimulated desorption. Chlorine adsorbs at 310 K to form a series of surface structures for increasing coverage, (root 3x root 3)R30 degrees-->(6 root 3x6 root 3)R30 degrees, all of which can be represented by a single matrix: [GRAPHICS] where x=2/3-->7/9. For 310 K adsorption and the pressures, P(Cl-2)approximate to 2x10(-9) mbar, used here no detectable reaction occurs after completion of the 6 root 3 structure. The activation energy for desorption for this chemisorbed surface is similar to 250 kJ mol(-1). Chlorine adsorption at 110 K initially follows the same sequence of structures as for 310 K adsorption, but beyond the 6 root 3 structure a corrosion reaction occurs which forms a thick layer of copper chloride, probably CuCl. The mechanism by which this chloride is formed is not clear, but a high concentration of physisorbed chlorine appears to be necessary. On heating the copper chloride covered surface some surface species are desorbed below 150 K, yielding molecular chlorine, CuCl and CuCl2. However, the majority of the copper chloride does not disappear from AES detection until 240-270 K and does so without being detected in the gas phase. It is thought that the majority of the chloride coalesces into crystallites while a minority decomposes with the chlorine diffusing into the copper crystal. The mean free paths of 62, 181 and 916 eV electrons through the copper chloride were estimated to be 4, 10 and 11 Angstrom, respectively. Chlorine from the chemisorbed (6 root 3x6 root 3)R30 degrees structure undergoes electron stimulated desorption under a 2 keV, 100-150 A m(-2) focussed electron beam. This property has been used to study the surface diffusion of chlorine on Cu(111) by monitoring the rate of depletion of chlorine under the beam with respect to time and temperature using AES. The activation energy to surface diffusion is found to be 19+/-2 kJ mol(-1) with a pre-exponential of (7+/-4) x 10(-7) m(2) s(-1).